Audio/video separator

ABSTRACT

An audio/video separator provides a high-performance and cost-effective solution to analog TV reception with only one A/D converter and a minimum of analog IF components. The apparatus may operate on a digitized TV signal and, when integrated with a digital video processor, process video signals while separating audio signals. The resultant audio and video signals may be considered to have excellent signal quality due to highly optimized demodulation architecture and digital signal processing techniques on both audio and video data paths.

FIELD OF THE INVENTION

The present invention relates to digital processing of analog televisionsignals and, more particularly, to separating audio and video signalsfrom an analog television signal.

BACKGROUND

Transmitting a basic television signal adhering to the NationalTelevision System Committee (NTSC) standard requires sending an audiosignal to accompany a video signal by frequency-modulating an audiocarrier located 4.5 MHz above a video carrier.

Conventional analog television (TV) receivers employ analog componentsto separate the audio signal from a received NTSC TV signal beforedemodulating and decoding the audio signal. Such analog componentsinclude surface acoustic wave (SAW) filters, amplifiers, automatic gaincontrol (AGC) circuitry and automatic frequency control (AFC) circuitry.

As is known in TV receiver design, a TV signal is received in the radiofrequency (RF) portion of the electromagnetic spectrum and converted toan intermediate frequency (IF) by components collectively referred to asa front end tuner. The video signal and audio signal are then,typically, separated at the output of the front end tuner, that is,while the TV signal is in the IF. Subsequently, each signal (audio,video) is demodulated separately by using separate filters andamplifiers.

It is known to perform the demodulation in the digital domain to takeadvantage of digital signal processing techniques. However, because suchdemodulation is signal specific, the TV signal is typically filtered bydistinct analog filters to simultaneously yield an audio signal and aseparate video signal. The separated audio signal and video signal arethen processed by corresponding analog-to-digital converters beforeseparate digital demodulation.

The disadvantages of using analog components to separate the audiosignal from the received NTSC TV signal are numerous. The analogapproach to TV receiver design may be considered to compromiseperformance due to a lack of precise analog signal filter, analogdetector and analog control circuit design. In particular, poorseparation of the audio signal and the video signal may cause mutualinterference between demodulated audio and video signals. Furthermore,reducing power consumption of the TV receiver may be difficult.

Clearly, an improved design is required for a receiver of an analogtelevision signal, where the improved design provides more preciseseparation of the audio signal and the video signal from the received TVsignal.

SUMMARY

An audio/video separator is implemented to separate a digitized audiosignal and a digitized video signal from a digitized version of ananalog television signal. Working in the digital domain, the audio/videoseparator provides higher-performance and lower cost than that of analogseparation solutions. Due to the precision of a digital filter employedto perform the separation of the audio and video signals from thedigitized television signal, the input to the filter should be as closeto baseband as possible. To this end, a video carrier recovery circuitmay be employed to determine, and act to reduce, any frequency offsetpresent in the digitized television signal. Once the frequency offsethas been reduced, a selection circuit may be employed to reverse theorder of video carrier recovery and filtering. Such a reversal of ordermay be seen to improve the output digitized audio signal and outputdigitized video signal, as a steady-state phase error introduced by thevideo carrier recovery circuit is then absent from the output audiosignal.

Optionally, the audio/video separator may be integrated with a digitalvideo processor. Since such an integrated digital processor andaudio/video separator operates with digital signal processingtechniques, many analog components associated with an analog approach tothe same tasks may be eliminated, such as SAW filters, amplifiers, AGCsand AFCs. In addition, the integrated digital processor and audio/videoseparator may be seen to offer superior video and audio performance dueto optimized digital filter design and signal processing algorithms.Finally, the integrated digital processor and audio/video separator maybe seen to consume less power than conventional analog receivers, inpart due to implementation as a Very Large Scale Integrated (VLSI)Circuit.

In accordance with an aspect of the present invention there is providedan apparatus for processing a near-baseband, received digitizedtelevision signal. The apparatus includes a video carrier recoverycircuit, a filter circuit and a selection circuit. The video carrierrecovery circuit is adapted to receive a video carrier recovery circuitinput signal, the video carrier recovery circuit input signal includinga video carrier signal, detect a phase offset of the video carriersignal, generate a phase adjustment signal based on the phase offset andproduce a video carrier recovery circuit output signal from the videocarrier recovery circuit input signal and the phase adjustment signal.The filter circuit is adapted to receive a filter circuit input signal,the filter circuit input signal including components in a frequencyrange that is expected to contain a digitized audio signal and produce afilter circuit output signal excluding the components in the frequencyrange. The selection circuit is adapted to switch between a firstconfiguration, wherein the near-baseband, received digitized televisionsignal is the video carrier recovery circuit input signal and the videocarrier recovery circuit output signal is the filter circuit inputsignal, and a second configuration, wherein the near-baseband, receiveddigitized television signal is the filter circuit input signal and thefilter circuit output signal is the video carrier recovery circuit inputsignal.

In accordance with another aspect of the present invention there isprovided a method of processing an analog television signal. The methodincludes converting the analog television signal to a digitizedtelevision signal having a frequency offset relative to baseband,reducing the frequency offset to produce a near-baseband digitizedtelevision signal having a residual frequency offset, producing a signalrepresentative of the residual frequency offset, based on the signalrepresentative of the residual frequency offset, reducing the frequencyoffset to produce a nearer-to-baseband digitized television signal andfiltering the nearer-to-baseband digitized television signal to producea first filter output signal having components restricted to a firstfrequency range, where the first frequency range is expected to containan audio signal.

In accordance with a further aspect of the present invention there isprovided a television signal reception system. The system includes atuner adapted to shift an analog television signal associated with aradio frequency carrier to an analog television signal at anintermediate frequency, an analog to digital converter adapted toproduce a digitized television signal having a frequency offset relativeto baseband, where the digitized television signal is based on theanalog television signal at the intermediate frequency, and a carrierseparator. The carrier separator includes a video carrier recoverycircuit adapted to detect the frequency offset and produce an indicationof the frequency offset, a mixer, responsive to receipt of theindication of the frequency offset, adapted to reduce the frequencyoffset to produce a near-baseband digitized television signal, and afilter adapted to filter the near-baseband digitized television signalto produce a digitized sound signal.

In accordance with a still further aspect of the present invention thereis provided an apparatus for processing a near-baseband, receiveddigitized television signal. The apparatus includes a video carrierrecovery circuit, a filter circuit and at least two selection switches.The video carrier recovery circuit includes a video carrier recoverycircuit input port to receive a video carrier recovery circuit inputsignal, the video carrier recovery circuit input signal including avideo carrier signal, a phase error detector to detect a phase offset ofthe video carrier signal, a loop filter to generate a phase adjustmentsignal based on said phase offset and a video carrier recovery circuitoutput port to produce a video carrier recovery circuit output signalfrom the video carrier recovery circuit input signal and the phaseadjustment signal. The filter circuit includes a filter circuit inputport to receive a filter circuit input signal, the filter circuit inputsignal including a frequency range that is expected to contain adigitized audio signal, a filter to produce a filter circuit outputsignal by excluding the components in the frequency range and a filtercircuit output port to transmit said filter circuit output signal. Afirst selection switch is adapted to receive first received signalsincluding the near-baseband, digitized television signal and the filtercircuit output signal and pass one of the first received signals to thevideo carrier recovery circuit input port. A second selection switch isadapted to receive second received signals including the near-baseband,digitized television signal and the video carrier recovery circuitoutput signal and pass one of the second received signals to the filtercircuit input port.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate example embodiments of this invention:

FIG. 1 illustrates a TV signal reception system including an integrateddigital demodulator and audio/video separator according to an embodimentof the present invention;

FIG. 2 illustrates, as a detailed block diagram, the integrated digitaldemodulator and audio/video separator from the system of FIG. 1according to an embodiment of the present invention;

FIG. 3 illustrates an audio/video separation filter for use in theintegrated digital demodulator and audio/video separator from the systemof FIG. 1 according to an embodiment of the present invention;

FIG. 4 illustrates a digital automatic gain controller for use in theintegrated digital demodulator and audio/video separator from the systemof FIG. 1 according to an embodiment of the present invention;

FIG. 5 illustrates a video carrier recovery circuit for use in theintegrated digital demodulator and audio/video separator from the systemof FIG. 1 according to an embodiment of the present invention;

FIG. 6A illustrates a portion of the block diagram of FIG. 2 in a firstconfiguration;

FIG. 6B illustrates a portion of the block diagram of FIG. 2 in a secondconfiguration; and

FIG. 7 illustrates an exemplary design for a front-end automatic gaincontrol unit from the integrated digital demodulator and audio/videoseparator of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates a TV signal reception system 100 for use, forexample, in a TV set or set-top box application. A standard TV antenna101 connects to a tuner 102. The output of the tuner is received by aVariable Gain Amplifier (VGA) 104 whose gain is selected based on areceived gain control signal whose origin will be discussed hereinafter.The TV signal at the output of the VGA 104 is then received by ananalog-to-digital (A/D) converter 106.

The A/D converter 106 has been implemented as the well-knownsample-and-hold type but, as will be appreciated by a person skilled inthe art, other types of A/D converters may be used here equally. Giventhat a common input IF TV signal with 6 MHz bandwidth from the tuner 102is centered at around 44.25 MHz, the A/D converter 106 may operate in abandpass sampling mode with typical sampling frequency around 25 MHz.The bandpass sampling effect of the A/D converter 106 is to frequencyshift the TV signal down to 6.25 MHz while digitizing the entire 6 MHzband of the desired TV channel.

An integrated digital demodulator and audio/video separator (IDD/AVS)108, whose structure will be described in detail hereinafter, receivesthe output of the A/D converter 106 and produces three outputs: adigital sound signal, which may also be referred to as a digital “soundIF” (SIF); a digital Composite Video Baseband Signal (CVBS); and a gaincontrol signal. The digital SIF is received by an audiodemodulator/decoder 110. The digital CVBS is received by a digital videodecoder 112 and a digital-to-analog (D/A) converter 114. The gaincontrol signal is received by the VGA 104. The output of the D/Aconverter 114 is received by an analog video decoder 116.

FIG. 2 is a detailed block diagram of the IDD/AVS 108. A basebanddown-converter 202, which may take the form of a controllable mixer,receives the input signal to the IDD/AVS 108. The output signal from thedown-converter 202 is received by a digital automatic gain control (AGC)unit 204.

The digital AGC unit 204 communicatively connects to a video carrierrecovery circuit 210 via a first multiplexer 208 and to an audio/videoseparation filter 214 via a second multiplexer 212.

An optimized, complementary, finite impulse response (FIR) digitalfilter may be used for the A/V separation filter 214. As illustrated inFIG. 3, the input to the A/V separation filter 214 is received at afilter input port 302 and split into two signal paths. The input signalon a first signal path is subjected to a low pass filter 304 with acut-off frequency configured such that the low pass filter passes thevideo signal components of the input signal to a video output port 306.The input signal on a second signal path is subjected to a high passfilter 308 that is complementary to the low pass filter 304 (i.e., thelow pass cut-off frequency of the low pass filter and the high passfilter are identical) and has a cut-off frequency configured to pass theaudio signal components of the input signal to an audio output port 310.

As mentioned hereinbefore, an audio signal accompanying a video signalin a television signal may be sent, according to the NTSC broadcastingstandard, by frequency-modulating an audio carrier located 4.5 MHz abovea video carrier. More particularly, when the television signal isshifted to baseband, the video signal will occupy frequencies from 0 to4.2 MHz and the audio signal will be centered around a 4.5 MHz centerfrequency and occupy around 120 KHz on each side of the 4.5 MHz centerfrequency. The applicant has successfully used a cut-off frequency ofaround 4.25 MHz in the two filters 304, 308 of the A/V separation filter214. As will be clear, the cut-off frequency is a programmable designcharacteristic of the two filters 304, 308 and, for the exemplary caseof an NTSC television signal, success may be realized using a cut-offfrequency in the range from 4.2 to 4.35 MHz.

In designing the two filters of the A/V separation filter 214 (FIG. 2),a minimum requirement of −60 dB was set for the stop band attenuation.Additionally, the passband ripple should be lower than 0.5 dB. To meetthese requirements, many possible designs were available. The chosen FIRfilter uses a least-squares error minimization method and has a 192-tapfilter length.

The video carrier recovery circuit 210 and the A/V separation filter 214connect to a video processor 230 through a third multiplexer 216. Themultiplexers 208, 212, 216 may be toggled between two states by amultiplex control (MUX CONTROL) signal. As they may be toggled betweentwo states, the multiplexers 208, 212, 216 may be considered selectionswitches. Furthermore, the A/V separation filter 214 is connected to thevideo carrier recovery circuit 210 via the first multiplexer 208. Thevideo carrier recovery circuit 210 is connected to the down-converter202 for the transmission of a frequency control signal.

As is known, a sync tip is the peak of the horizontal synchronizationpulse. In the video signal, only the sync tip parts (roughly 11 microsecond long for each horizontal line) are deterministic, while thevisible parts (roughly 53 micro seconds) are random. To perform any kindof AGC (including that controlled by the front-end AGC unit 206), it isnecessary to depend on the deterministic part of the video signal, thatis, the sync tip part.

The front-end AGC unit 206 receives the same input signal as thedown-converter 202 in addition to the information on sync tip positionreceived from the video processor 230 and sends the gain control signalto the VGA 104 (FIG. 1).

FIG. 4 illustrates an exemplary structure for the digital AGC unit 204.A signal being output from the digital AGC unit 204 is assessed by adetector 402. An indication of the value of a detected metric may thenbe passed to an error calculator 404. An error signal may be passed fromthe error calculator 404 to a limiter 406. A limited error signal maythen be passed to a loop filter 408. The signal incoming to the digitalAGC unit 204 may then be multiplied by the filtered error signal at amultiplier 410.

An exemplary structure for the video carrier recovery circuit 210 isillustrated in FIG. 5. A signal incoming to an input port 501 of thevideo carrier recovery circuit 210 is split into two paths. In the first(lower) path, the signal is received by a low pass filter 502. Theoutput of the low pass filter 502, which may, for instance, passfrequencies from 0 to 500 KHz, is passed to a phase error detector 506via a first multiplier (or mixer) 504. A phase error output from thephase error detector 506 is then received by a loop filter 508. The loopfilter 508 produces a frequency offset for sending to the down-converter202 (for gross frequency offset control) and an instantaneous phaseadjustment signal for sending to the first multiplier 504 (for phaseerror control, which may be considered, over time, to be analogous tofine frequency offset control). In the second (upper) path, the signalreceived by the input port 501 is passed to a second multiplier (ormixer) 510, which also receives the instantaneous phase adjustmentsignal. The output of the second multiplier 510 is passed to an outputport 511 as the output of the video carrier recovery circuit 210. Theoperation of the video carrier recovery circuit 210 may be consideredanalogous to the operation of a conventional phase locked loop,including the phase error detector 506, the loop filter 508 and aphase-controllable signal source, wherein the combination of the inputport 501, the low pass filter 502, the first multiplier 504 and thesecond multiplier 510 may be considered the phase-controllable signalsource.

A sync tip position determined by the video processor 230 is received bythe front-end AGC unit 206, whose components are illustrated in FIG. 7.In particular, the sync tip position is received by a squaring unit 702,which also receives a digitized TV signal from the digitized A/Dconverter 106. The output of the squaring unit is received by alogarithmic unit 704 whose corresponding output is received by adifference detector 706. A power target is also received by thedifference detector 706 such that a difference between the output of thelogarithmic unit 704 may be compared to the power target and thedifference forwarded to a loop filter 708. The output of the loop filter708 is the gain control signal used to control the VGA 104 (FIG. 1).

In overview, rather than separate the audio signal and the video signalfrom an IF TV signal using analog components, the analog IF TV signal isdigitized and the digitized TV signal is processed using digitalcomponents. As a result, more accurate audio and video signals areavailable at the output and cost savings may be realized.

In operation, the antenna 101 receives a broad spectrum ofelectromagnetic radiation and passes an electrical signal representativeof that radiation to the tuner 102. The tuner 102 tunes to a specificfrequency range, i.e., a TV channel, in the RF part of theelectromagnetic radiation spectrum. The result of such tuning is afrequency shift of the RF TV signal to an IF TV signal. The IF TV signalis then filtered and amplified by the VGA 104.

In particular, the VGA 104 may either boost or attenuate the received IFTV signal such that the input (peak) power to the A/D converter 106 ismaintained at a desirable level for sampling, regardless of the signalpower received at the antenna. An IF strip (not shown), as part of theVGA 104, may include anti-aliasing filters, which are useful forpreparing the IF TV signal for sampling and for attenuating interferingadjacent signals. The VGA 104 may be controlled in a closed-loop fashionby the front-end AGC unit 206 (FIG. 2) in a manner that will bedescribed hereinafter.

The filtered and amplified IF TV signal is then digitized by the A/Dconverter 106 to result in a digitized TV signal. The digitized TVsignal output from the A/D converter 106 is received by the IDD/AVS 108,which, as mentioned hereinbefore, produces a digital CVBS, a digital SIFand a GC signal.

The digital CVBS may be processed by the digital video decoder 112 toproduce desirable digital video output, for instance, digital video overan interface adhering to the International Telecommunication Union (ITU)standard ITU-R BT.686. Alternatively, the digital CVBS may be convertedto an analog CVBS by the digital-to-analog (D/A) converter 114 and theanalog CVBS processed by the analog video decoder 116 to producedesirable analog video output, for instance, in the known RGB format orthe known YPrPb format.

The digital SIF may be processed by the audio demodulator/decoder 110 toproduce desirable digital audio output, for instance, digital audio overan I2S interface. As will be familiar to the person skilled in the art,I2S is a digital audio interface used inside equipment to transfer audiobetween integrated circuits.

Operation of the IDD/AVS 108 may be better understood in view of FIG. 2.The down-converter 202 mixes the received digitized TV signal such thatthe frequency offset of the video carrier is reduced from 6.25 MHz downto near baseband (0 MHz, i.e., direct current or “DC”) with a fixedconversion ratio. The down-converter 202 is further adapted to reduce aremaining frequency offset, an indication of which may be received fromthe video carrier recovery circuit 210. The down-converter 202 mayinclude a sharp, brick-wall, linear-phase low pass filter (not shown) toattenuate interfering signals outside the frequency band of interest,thereby removing adjacent channel and image interference. Morespecifically, application of the brick-wall filter may be delayed untilit has been determined, and indicated to the down-converter 202 by theloop filter 508 of the video carrier recovery circuit 210, that thefrequency offset of the digitized TV signal has been significantlyreduced.

Once the frequency offset of the video carrier that characterizes thedigitized TV signal has been significantly reduced, it may be consideredthat the low pass filter of the down-converter 202 sufficiently filtersout adjacent channel interference. Accordingly, after the down-converter202, only the desired signal remains. The level of the desired signalcan be very small. Ideally, in a fixed-point implementation, the desiredsignal should occupy the top bits, i.e., the desired signal should berepresented by the full range of bits available. Such a full rangerepresentation is known to reduce quantization error in subsequentprocessing. The digital AGC unit 204 acts to boost the level of thedesired signal after the down-converter 202.

Thus the digital AGC unit 204 may be used to counter some of the effectsof adjacent channel interference. Assuming a desired channel is channel6, it may be that the signal at channels 5 and 7 are very strong.Adjacent channel interference signals from channels 5 and 7 may bereceived at the A/D converter 106. If the adjacent channel interferenceis sufficiently stronger than the desired signal (channel 6), then themajority of the A/D range may be occupied by the interference.Consequently, the desired signal may only occupy a small part of the A/Drange.

A signal being output from the digital AGC unit 204 is assessed by thedetector 402 (FIG. 4). According to the received AGC control signal, thedetector 402 may detect the value of a metric of the signal being outputfrom the digital AGC unit 204, where the metric may be, for instance,peak amplitude or average power. An indication of the value of thedetected metric may then be passed to the error calculator 404. Inaddition to indicating which metric to detect (e.g., peak amplitude,average power, etc.), the AGC control signal may also indicate a targetvalue for that metric to the error calculator 404. The error calculator404 may perform a comparison of the detected value of the metric withthe target value of the metric to produce an error signal. The errorsignal may then be passed to the limiter 406, which acts to generate alimited error signal in which small errors are zeroed out. The limitederror signal is then passed to the loop filter 408. The loop filter 408filters the limited error signal to produce a filtered error signal. Theloop filter 408 may be provisioned with a very small bandwidth to avoidintroducing noise. The signal incoming to the digital AGC unit 204 isthen multiplied by the filtered error signal at the multiplier 410,i.e., the signal incoming to the digital AGC unit 204 is amplified bythe multiplier 410 with a gain based on the filtered error signal.

The video carrier recovery circuit 210 may operate primarily in twomodes. In an initial (acquisition) mode, the video carrier recoverycircuit 210 determines an initial amount of frequency offset (frombaseband) in the near-baseband digitized TV signal received from thedigital AGC unit 204 by detecting a video carrier signal within thedemodulated channel. Notably, where the down-converter 202 provides aninitial frequency shift of the order of 6.25 megahertz, the videocarrier recovery circuit 210 may instruct, in the acquisition mode, thedown-converter 202 to perform a further frequency shift of the order ofhundreds of kilohertz. The output of the video carrier recovery circuit210 in the acquisition mode is a feedback signal indicating the initialamount of frequency offset to the down-converter 202, so that theinitial amount of frequency offset may be removed from the digitized TVsignal.

In a subsequent (tracking) mode, the video carrier recovery circuit 210may track a residual (usually small) frequency offset to ensure that thespectrum of the output digital TV signal is precisely positioned in DCprior to a video/audio signal separation operation and/or subsequentvideo demodulation. When the spectrum of the output digital TV signal isproperly positioned, the video carrier may be perceived only as a DClevel, rather than a sinusoid having a frequency. Residual frequencyoffset, if any, is often primarily due to phase noise at the tuner 102.The output of the video carrier recovery circuit 210 in the trackingmode is the digitized TV signal with both the initial amount offrequency offset removed and the residual amount of frequency offsetsignificantly reduced. To reduce the residual amount of frequencyoffset, the video carrier recovery circuit 210 acts to adjust the phaseof the output signal in accordance with a difference between the phaseof the video carrier and a reference phase. It is known that the outputof a phase locked loop, to which, as discussed hereinbefore, the videocarrier recovery circuit 210 may be considered analogous, will alwayshave some phase jitter. As should be clear to a person skilled in theart, the extent of the phase jitter in the output digital TV signal fromthe video carrier recovery circuit 210 is directly related to the amountof bandwidth for which the loop filter 508 (see FIG. 5) is configured.

More detailed operation of the video carrier recovery circuit 210 may beconsidered in view of the exemplary structure illustrated in FIG. 5. Inthe first path, the low pass filter 502 removes high frequencycomponents of the signal received by the video carrier recovery circuit210. The output of the low pass filter 502 is passed to the phase errordetector 506 via the first multiplier 504. If the digitized TV signal isproperly positioned at DC, the output of the low pass filter is onlyreal valued. The phase error detector 506 compares the phase of theoutput of the first multiplier 504 to a reference phase (i.e., zero) togenerate a detected phase error. If the digitized TV signal is notproperly positioned at DC, the output of the low pass filter is complexvalued, and therefore has a non-zero phase. A signal representative ofthe detected phase error is passed by the phase error detector 506 tothe loop filter 508. The loop filter 508 processes the signalrepresentative of the detected phase error and, along with smoothing anysudden changes in the signal representative of the detected phase error,determines a magnitude of a phase adjustment, θ, to counteract the phaseerror.

Such phase error control, during which a phase error of small magnitudeis corrected by sending an instantaneous phase adjustment signal,e^(iθ), to the first multiplier 504 and the second multiplier 510, maybe seen, over time, to constitute fine frequency error control.

For gross frequency error control, an ongoing phase error of largemagnitude (representative of a large frequency offset) is corrected bysending a frequency offset signal, determined as the rate of change ofthe phase adjustment, θ, to the down-converter 202 as a valuerepresentative of the large frequency offset.

The output digital TV signal of the video carrier recovery circuit 210in the tracking mode is the output of the second multiplier 510.

While the acquisition mode may be seen to be concerned with thereduction of large scale frequency offset (hundreds of KHz) at thedown-converter 202, the tracking mode may be seen to be concerned withthe reduction of phase error within the video carrier recovery circuit210, which has the consequence of reducing small scale frequency error.The loop filter 508 may determine that it is time to switch from theacquisition mode to the tracking mode, wherein the instantaneous phaseadjustment signal is sent to the first multiplier 504 and the secondmultiplier 510, when the magnitude of the frequency offset being sent bythe loop filter 508 to the down-converter 202 is smaller than apre-defined threshold. Alternatively, the loop filter 508 may switchfrom the acquisition mode to the tracking mode after a pre-defined timeof operation, say, 10 ms after power up.

Through a connection between the loop filter 508 and the output port511, the loop filter 508 may control whether a digitized TV signal is(tracking mode) or is not (acquisition mode) output from the videocarrier recovery circuit 210.

The acquisition mode, wherein frequency offset detection by the videocarrier recovery circuit 210 and compensation by the down-converter 202is required, may be returned to periodically for situations wherein thedigitized TV signal frequency drifts, for example, due to poorperformance of the tuner 102.

A frequency-offset reduced digitized TV signal may then be passed fromthe video carrier recovery circuit 210 to the A/V separation filter 214.Advantageously, the A/V separation filter 214, whose structure has beendiscussed hereinbefore, may be configured to provide appropriateaudio/video separation in addition to a linear phase transfercharacteristic so that subsequent Vestigial Sideband filtering (at thevideo processor 230) and audio demodulation (at the audiodemodulator/decoder 110 of FIG. 1) can be carried out on the resultantdigitized video output signal and digitized audio output signal (digitalSIF signal), respectively, without mutual interference.

As discussed hereinbefore, the IDD/AVS 108 can be re-configured througha multiplex control (MUX CONTROL) signal to reverse the signalprocessing order of the video carrier recovery circuit 210 and the A/Vseparation filter 214. Advantageously, a processing order in which theA/V separation filter 214 precedes the video carrier recovery circuit210 (FIG. 6B) minimizes interference on the audio signal that isintroduced due to the phase jitter present on the output of the videocarrier recovery circuit 210 in tracking mode. Any residual frequencyoffset on the audio signal becomes a DC component after the FMdemodulation and may be removed by notch filters after audio decoding.Conversely, a processing order in which the video carrier recoverycircuit 210 precedes the A/V separation filter 214 (FIG. 6A) avoids asituation in which an attempt is made to demodulate the audio signalbefore the majority of the frequency offset is removed. Such anattempted audio signal demodulation may lead to very noisy audio whilethe video carrier recovery circuit 210 is in the acquisition mode. As itis advantageous not to output a digitized TV signal in the acquisitionmode, the configuration of FIG. 6A may more accurately be illustratedwithout a connection between the video carrier recovery circuit 210 andthe A/V separation filter 214.

According to a first MUX CONTROL signal, the output of the firstmultiplexer 208 is the signal received at the input labeled 1.Furthermore, the output of the second multiplexer 212 is the signalreceived at the input labeled 1 and the output of the third multiplexer216 is the signal received at the input labeled 1. In this firstconfiguration, illustrated without the multiplexers 208, 212, 216 inFIG. 6A, the output of the digital AGC unit 204 is received by the videocarrier recovery circuit 210, whose output is passed to the A/Vseparation filter 214. The video output of the A/V separation filter 214is then passed to the video processor 230.

According to a second MUX CONTROL signal, the output of the firstmultiplexer 208 is the signal received at the input labeled 0.Furthermore, the output of the second multiplexer 212 is the signalreceived at the input labeled 0 and the output of the third multiplexer216 is the signal received at the input labeled 0. In this secondconfiguration, illustrated without the multiplexers 208, 212, 216 inFIG. 6B, the output of the digital AGC unit 204 is received by the A/Vseparation filter 214, whose video output is passed to the video carrierrecovery circuit 210. The output of the video carrier recovery circuit210 is then passed to the video processor 230.

While the video carrier recovery circuit 210 is in the tracking mode,the second configuration may be used so that the phase jitter in thevideo carrier recovery circuit 210 does not degrade the audio outputsignal from the A/V separation filter 214. Where the video carrierrecovery circuit 210 employs a high-bandwidth loop filter, phase jittermay be introduced into the both the video and audio output signals. Suchphase jitter, while tolerable in the video output signal, could bedetrimental to the subsequent demodulation and decoding of the digitizedaudio output signal. That is, for demodulation of the digitized audiosignal, a slight frequency offset is preferred to phase jitter. Suchthat the second configuration is used while the video carrier recoverycircuit 210 is in the tracking mode, the MUX CONTROL signal may begenerated by the loop filter 508 responsive to the switch between theacquisition mode and the tracking mode. The MUX CONTROL signal,generated in this way, would select a configuration corresponding to themode of the video carrier recovery circuit 210.

At the video processor 230, the output of the third multiplexer 216,i.e., a digitized video signal, is received and processed to produce adigital CVBS and a sync tip position using methods beyond the scope ofthe present application. The video processor 230 provides the sync tipposition to the front-end AGC unit 206.

The squaring unit 702 of the front-end AGC unit 206 (FIG. 7) receivesthe digitized TV signal that is also input to the down-converter 202 andan indication of sync tip position from the video processor 230. Thesquaring unit 702 is, in particular, concerned with the sync tip portionof the digitized TV signal, which is known to have the highest level inthe TV signal. As discussed hereinbefore, the sync tip portion of thedigitized TV signal is the only deterministic signal in the wholedigitized TV signal. While using the sync tip position to perfecttiming, the squaring unit 702 multiplies signal level of the inputsignal at the sync tip position by itself. A squared sync tip signallevel is then received by the logarithmic unit 704. A base-2 logarithmof the squared sync tip signal level is then produced by the logarithmicunit 704 and passed to the difference detector 706.

Effectively, the combination of the squaring unit 702 and thelogarithmic unit 704 produce a determined value proportional to thepower in the digitized TV signal. Such a proportional value may becompared, in the difference detector 706, to a target value for thepower. If the determined value is greater than the target value, thedifference detector 706 produces a gain control signal that will reducethe gain in the VGA 104. If the determined value is less than the targetvalue, the difference detector 706 produces a gain control signal thatwill increase the gain in the VGA 104. The gain control signal is sentto the VGA 104 via the loop filter 708, which acts to smooth any suddenchanges in the gain control signal.

Notably, rather than measuring power in the digitized TV signal, thefront-end AGC unit 206 may be configured to measure a different targetmetric. The target metric may be, for example, peak voltage. Broadlystated, the front-end AGC unit 206 produces gain control signals thatare sent to the IF VGA 104 (FIG. 1) to maintain a target metric at adesired level. The desired level is selected so that the analog IF TVsignal at the output of the VGA 104 is received optimally by the A/Dconverter 106, i.e., to avoid clipping (of signal that is too strong) ora signal that is too weak.

Alternatively or additionally, the front-end AGC unit 206 may sendcontrol signals to an amplifier within the tuner 102 (FIG. 1) tofacilitate delayed-AGC application.

In contrast to existing digital video demodulation solutions thatseparate the analog and video signals from an analog IF TV signal andthen digitize the analog audio signal and the analog video signalseparately, only one A/D converter (A/D converter 106) may be employedin the receiver design proposed herein for digitizing an entire TVsignal. The separation of the audio signal from the video signal maythen take place in the digital domain, which allows more accurateseparation than available in the analog domain.

The multi-mode video carrier recovery (acquisition, tracking) at thevideo carrier recovery circuit 210 coupled with the complementary designof the A/V separation filter 214 may be seen to improve audio/videoseparation and digital CVBS construction. As a result, the demodulateddigital CVBS has SIF rejection qualities and video performance (i.e.,signal-to-noise ratio) that represent a significant improvement overprevious designs, especially in the presence of a large frequencyoffset. In addition, the phase linearity of the complementary FIR filter(i.e., the A/V separation filter 214 of FIG. 2) provides for a reductionin harmonic distortion for the audio signal.

The ability, within the receiver design proposed herein, to reconfigurethe order in which frequency offset reduction (as controlled by thevideo carrier recovery circuit 210) and audio/video separation (at theA/V separation filter 214) are performed may be seen to further ease thedemodulation of the output digitized audio signal.

Although the majority of the preceding description is specific totelevision signals that follow the NTSC standard, it will be appreciatedby those skilled in the art, that the preceding may equally be appliedto television signals that follow an alternative format such as thePhase Alternating Line (PAL) format or the Systeme Electronique CouleurAvec Memoire (SECAM) format.

Other modifications will be apparent to those skilled in the art and,therefore, the invention is defined in the claims.

1. An apparatus for processing a near-baseband, received digitizedtelevision signal comprising: a video carrier recovery circuit adaptedto: receive a video carrier recovery circuit input signal, said videocarrier recovery circuit input signal including a video carrier signal;detect a phase offset of said video carrier signal; generate a phaseadjustment signal based on said phase offset; produce a video carrierrecovery circuit output signal from said video carrier recovery circuitinput signal and said phase adjustment signal; a filter circuit to:receive a filter circuit input signal, said filter circuit input signalincluding components in a frequency range that is expected to contain adigitized audio signal; produce a filter circuit output signal excludingsaid components in said frequency range; and a selection circuit adaptedto switch between: a first configuration, wherein said near-baseband,received digitized television signal is said video carrier recoverycircuit input signal and said video carrier recovery circuit outputsignal is said filter circuit input signal; and a second configuration,wherein said near-baseband, received digitized television signal is saidfilter circuit input signal and said filter circuit output signal issaid video carrier recovery circuit input signal.
 2. The apparatus ofclaim 1 wherein said filter circuit comprises a low pass finite impulseresponse digital filter.
 3. The apparatus of claim 1 wherein said videocarrier recovery circuit is further adapted to produce an indication ofa frequency offset and said apparatus further comprises a down-converteradapted to: receive an offset-from-baseband digitized television signal;receive said indication of said frequency offset from said video carrierrecovery circuit; and based on said indication of said frequency offset,frequency shift said offset-from-baseband digitized television signal bysaid frequency offset.
 4. The apparatus of claim 3 wherein said filtercircuit input signal includes further components in a second frequencyrange, said second frequency range expected to contain said digitizedvideo signal and wherein said filter circuit further comprises a secondfilter adapted to produce a second filter circuit output signalexcluding said further components in said second frequency range.
 5. Theapparatus of claim 4 wherein said second filter comprises a high passfinite impulse response digital filter.
 6. The apparatus of claim 5further comprising a video signal processor adapted to process saidfilter circuit output signal and produce a digital composite videobaseband signal according to a predetermined standard.
 7. The apparatusof claim 6 wherein said predetermined standard is the NationalTelevision System Committee standard.
 8. The apparatus of claim 6wherein said digitized video signal processor is adapted to determine aposition for a peak of a horizontal synchronization pulse for saiddigital composite video baseband signal.
 9. The apparatus of claim 8further comprising a front-end gain control circuit adapted to generatea gain control signal based on a characteristic of said digitizedtelevision signal and an indication of said position for said peak ofsaid horizontal synchronization pulse.
 10. A method of processing ananalog television signal comprising: converting said analog televisionsignal to a digitized television signal having a frequency offsetrelative to baseband; in a first mode of operation, reducing saidfrequency offset to produce a near-baseband digitized television signalhaving a residual frequency offset; producing a first mode residualsignal representative of said residual frequency offset of said firstmode near-baseband digitized television signal; based on said first moderesidual signal, reducing said frequency offset to produce a first-modenearer-to-baseband digitized television signal; filtering said firstmode nearer-to-baseband digitized television signal to produce a firstmode first filter output signal in a frequency range, expected tocontain an audio signal; in a second mode of operation, reducing saidfrequency offset to produce a second mode near-baseband digitizedtelevision signal having a residual frequency offset; filtering saidsecond mode near-baseband digitized television signal to produce asecond mode first filter output in a frequency range expected to containan audio signal, and a second mode second filter output signal, in afrequency range expected to contain a digitized video signal; producinga second mode residual signal representative of said residual frequencyoffset of said second mode near-baseband digitized television signalfrom said second mode second filter output signal; based on said secondmode residual signal, reducing said frequency offset to produce a secondmode nearer-to-baseband digitized television signal.
 11. The method ofclaim 10 further comprising filtering said first mode nearer-to-basebanddigitized television signal to produce a first mode second filter outputsignal having components restricted to a frequency range, expected tocontain a digitized video signal.
 12. The method of claim 11 furthercomprising demodulating said first mode second filter output signal toproduce a digital composite video baseband signal according to apredetermined standard.
 13. The method of claim 12 wherein saidpredetermined standard is the National Television System Committeestandard.
 14. A television signal reception system comprising: a tuneradapted to shift an analog television signal associated with a radiofrequency carrier to an analog television signal at an intermediatefrequency; a variable gain amplifier; an analog to digital converteradapted to produce a digitized television signal having a frequencyoffset relative to baseband, where said digitized television signal isbased on said analog television signal at said intermediate frequency; aseparator including: a video carrier recovery circuit adapted to detectsaid frequency offset and produce an indication of said frequencyoffset; a mixer, responsive to receipt of said indication of saidfrequency offset, adapted to reduce said frequency offset to produce anear-baseband digitized television signal; a filter adapted to filtersaid near-baseband digitized television signal to produce a digitizedsound signal; a second filter to filter said near-baseband digitizedtelevision signal to produce a digitized video signal; a video processoradapted to produce a digital component video baseband signal based onsaid digitized video signal; wherein said separator and said videoprocessor are further adapted to produce a control signal for saidvariable gain amplifier to maintain characteristics of said analogtelevision signal at said intermediate frequency within a rangeacceptable to said analog digital converter, where said control signalis based on said digital component video baseband signal.
 15. Thereception system of system of claim 14 wherein said control signal isbased in part upon a position for a peak of a horizontal synchronizationpulse in said digital component video baseband signal.
 16. The receptionsystem of claim 15 wherein said control signal is based in part upon atarget metric of said digital television signal at the output of saidanalog to digital converter.
 17. The reception system of claim 16wherein said target metric is peak amplitude.
 18. The reception systemof claim 16 wherein said target metric is peak power.
 19. An apparatusfor processing a near-baseband, received digitized television signalcomprising: a video carrier recovery circuit including: a video carrierrecovery circuit input port to receive a video carrier recovery circuitinput signal, said video carrier recovery circuit input signal includinga video carrier signal; a phase error detector to detect a phase offsetof said video carrier signal; a loop filter to generate a phaseadjustment signal based on said phase offset; a video carrier recoverycircuit output port to provide a video carrier recovery circuit outputsignal from said video carrier recovery circuit input signal and saidphase adjustment signal; a filter circuit including: a filter circuitinput port to receive a filter circuit input signal, said filter circuitinput signal including components in a frequency range that is expectedto contain a digitized audio signal; a filter to produce a filtercircuit output signal by excluding said components in said frequencyrange; and a filter circuit output port to transmit said filter circuitoutput signal; a first selection switch to receive first receivedsignals including said near-baseband, digitized television signal andsaid filter circuit output signal and pass one of said first receivedsignals to said video carrier recovery circuit input port; and a secondselection switch to receive second received signals including saidnear-baseband, digitized television signal and said video carrierrecovery circuit output signal and pass one of said second receivedsignals to said filter circuit input port.